Combined telephone-telegraphy system wherein local calling box includes telephone equipment and telegraphy transmitter



May 21, 1968 F. E. WELD ETAL 3,384,714

COMBINED TELEPHONE-TELEGRAPHY SYSTEM WHEREIN LOCAL CALLING BOX INCL UDESTELEPHONE EQUIPMENT AND TELEGRAPHY TRANSMITTER Filed Oct. 12, 1964 4Sheets-Sheet 1 I i 22 3o I nsconnea I mmcnons 'rc REMOTE LOCATION I l II vouce l LOGIC I 3z 36 i i vonc: VOICE l 26 A rnmsmr'rm RECEIVER "4/20DECODERS v I L.,/ 1 I 24 W -as i l y L J l4 POWER BOX SUPPLY f souncs l6lo/ BOX 2 i --l BOX N1F---- sex a z ,(202 I Z 204 4 20a 2|2 2lO 232 FIG,4

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v v ROBERT w. LASSELL a Q ROBERT B. McLEOD ATTORNEYS y 21, 1968. F. E.WELD ETAL 3,384,7

COMBINED TELEPHONE'TELEGRAPHY SYSTEM WHEREIN LOCAL CALLING BOX INCLUDESTELEPHONE EQUIPMENT AND TELEGRAPHY TRANSMITTER Filed OCT. 12, 1964 4Sheets-Sheet 3 g I v I a I I I m I I T L I 5 5 5: E E I Q l a a g I 3 5l r a I 8 S i I I 5 2*: I E g E 5 I L. 3 g 6 3' I N INVENTORS- if E 8 8FOSTER E. WELD,

l l i ROBERT w. LASSELL a i v ROBERT E. McLEOD L j W8 6001/ ATTORNEYSMay 21, 1968 F. E. WELD ETAL COMBINED TELEPHONE-TELEGRAPHY SYSTEMWHEREIN LOCAL CALLING BOX INCLUDES TELEPHONE EQUIPMENT AND TELEGRAPHYTRANSMITTER 4 Sheets-Sheet 3 Filed Oct. 12, 1964 8 w mwo TD E E flmd M oWw & m m w h &8 M 358 RTT 5&3 ERR l 988 i mmmT v1 F 02/ 09 M J4 nu m TAQI W3: 55.. 523mm E wt 8E :05 36 L: L pm: m: flow 2228 m P52: 55.. SSE85. :3 mm: =2: T1 i I L 00- mm;

NS m2 m2 1w m 5.13%. 551 5.5.555 36 $3 :2: 36 Q3 32 0! NB mm 4Sheets-Sheet 4 y 1968 F. E. WELD ETAL COMBINED TELEPHONE'TELEGRAPHYSYSTEM WHEREIN LOCAL CALLING BOX INCLUDES TELEPHONE EQUIPMENT ANDTELEGRAPHY TRANSMITTER Filed Oct. 12, 1964 INVENTORS.

XEP EA @258 A T r Sta wm N mmm I3 :5 5:3 5:: mN V wmm 0mm wvm E92 m N228 \l. AT =2 .23 5 mm vw 02 E350 E350 5.2.6 .5350 N2 m mm m f N 2 az zoz wm F 3 7 7 a 8. r 5 Q9 m2 :2 i N .550 Isr T2? T2? Ls P522. 55085 2E033 $830 5253 E583 FOSTER E. WELD, ROBERT w. LASSELL a ROBERT E. McLEODBY 5, Body ATTORNEYS United States Patent 0 COMBINEDTELEPHONE-TELEGRAPHY SYSTEM WHEREIN LOCAL CALLING BOX INCLUDES TELEPHONEEQUIPMENT AND TELEGRAPHY TRANSMITTER Foster E. Weld, Newton Highlands,Robert W. Lassell,

Westwood, and Robert B. McLeod, Clinton, Mass, assignors to E. W. BlissCompany, Canton, Ohio Filed Oct. 12, 1964, Ser. No. 403,316 14 Claims.(Cl. l79--3) ABSTRACT (IF THE DISCLOSURE A combined telephone-telegraphysystem is disclosed herein and which includes a plurality of localcalling boxes and central station communication equipment connectedtogether in a series circuit. Each local calling box includes telephoneequipment for transmitting and receiving voice frequency signals to andfrom the central station equipment, as well as telegraphy means fortransmitting coded box, identifying tone frequency signals to thecentral station equipment. Each local calling box also includes meansfor providing electrical power for the local calling box from currentflow in the series circuit. The central station equipment includestelephone equipment for transmitting and receiving voice frequencysignals to and from the local calling boxes, as well as means responsiveto the coded box identifying tone frequency signals for identifying theparticular box transmitting the coded box identifying signal.

This invention pertains to the art of communications and moreparticularly to a combined telephone-telegraphy communication system.

The invention is particularly applicable in conjunction with fire alarmand police call systems and will be described with particular referencethereto, but is not limited to same as it may be used for variousapplications in which combined telephone and telegraphy communicationsare desired.

A typical fire alarm system takes the form of a series 100p connectingvarious boxes, i.e., local fire alarm boxes, together in a currentcarrying series circuit with central oifice equipment. A fire alarm callmay be sent from each of the series connected boxes to the centralstation by merely actuating a lever at the box, which in turn actuates asuitable signaling device, which interrupts the current flow in adistinctive manner representative of the calling box.

Frequently it is desirable that in addition to the distinctive signalidentifying the signaling box, a telephone capability be providedbetween the calling box and the central station in order that a personreporting a fire may communicate by voice with the fire departmentpersonnel to, for example, explain the extent of the fire. Also, if sucha combined telephone-telegraphy fire alarm system be provided it isdesirable that a distinctive coded signal identifying the calling boxhave priority, i.e., be received first, over voice communication so thatthe voice communications will never interfere with or delay fire alarmreception. Further, due to the time proven reliability of conventionalseries connected fire alarm boxes, it is desirable that such atelephone-telegraphy fire alarm system utilize wherever possible theexisting wire facilities thereby alleviating the necessity of installingnew metallic interconnecting circuits.

In addition to the foregoing reasons for improving fire alarm systems,municipalities also find it desirable to utilize existing fire alarmcircuits for police call reporting systems. That is, it may be desirablefor a patrolman to 3,384,?14 Patented May 21, 1968 utilize a telephonelocated at a fire alarm box for reporting to police headquarters over amunicipally owned communication network.

The present invention is directed toward means for satisfying theforegoing needs of municipalities, as well as others who desire acombined telephone-telegraphy system.

In accordance with the present invention there is provided a combinedtelephone-telegraphy system including a plurality of local calling boxesand central station equipment connected together in a series circuit andadapted to be connected across a power supply source for maintainingcurrent flow in the series circuit. Each of the local calling boxesincludes voice communication means for transmitting and receiving voicefrequency signals to and from the central station equipment, as well astelegraphy means for transmitting coded box identifying frequencysignals to the central station equipment. The central station equipmentincludes voice communication means for transmitting and receiving voicefrequency signals to and from the local calling boxes, as well ascalling box identifying means responsive to the coded box identifyingfrequency signals for identifying the particular calling boxtransmitting the coded box identifying signals. In addition, each of thelocal calling boxes includes circuit means for providing operating powerfor the box from the current flowing in the series circuit.

In accordance with another aspect of the present invention, there isprovided a novel calling box including communication means fortransmitting and receiving voice frequency signals, as well astelegraphy means for transmitting coded box identifying signals. Inaddition, circuit means are provided in the calling box adapted to beconnected to a current carrying conductor for develop ing box operatingpower therefrom. The circuit means may include power supply means fordeveloping bias potential for supplying bias potential to the voicecommunication means, as well as to the telegraphy means.

In accordance with a still further aspect of the present invention,normally closed switching means are provided for shunting the circuitmeans in said local calling box thereby rendering the circuit meansnonresponsive to current flow in the conductor until the switching meansis opened. Further, said circuit means may include a Zener diodeconnected across the normally closed switching means in such a mannerwith respect to the direction of the current flow in the conductor thatwhen the switching means is opened the Zener diode will be reversedbiased.

In accordance with a still further aspect of the present invention, thetelegraphy means Within each box includes oscillator means fordeveloping a coded frequency signal representative of the calling box.In addition, the calling box may include the time delay circuit meansfor deactivating the oscillator means after a predetermined timesubsequent to the opening of the normally closed switching means wherebythe coded frequency signal s transmitted for a predetermined period.

In accordance with a still further aspect of the present invention, thecalling box includes common power amplifying means for amplifying theoutput of the voice communication transmitting means, as well as theoutput of the telegraphy means.

In accordance with a still further aspect of the present invention,novel central station equipment means are provided adapted for use inreceiving coded box identifying frequency signals from a plurality ofcalling boxes and identifying which calling box transmitted the codedbox identifying signals. The central station equipment means includesvoice communication means for transmitting and receiving voice frequencysignals, as well as calling box identifying means responsive to receivecoded box identifying frequency signals for identifying which callingbox transmitted coded box identifying signals. Still further, thecentral station equipment means includes signal separation circuit meansfor purposes of separating received voice frequency signals from thecoded box identifying frequency signals and respectively coupling thesignals to the central station voice communication means and to thecalling box identifying means.

In accordance with a still further aspect of the present invention, thebox identifying means in the central s ation equipment means includes aplurality of decoder means each responsive to a particular frequencywithin a frequency range of signals passed by a band pass filter meansincluded in the signal separation circuit means for purposes ofproviding an output decoder signal.

In accordance with a still further aspect of the present invention, thecentral station equipment means includes logic circuit means responsiveto various combinations of the output decoder signals for determiningwhich local calling box is transmitting coded box identifying frequencysignals.

The primary object of the present invention is to provide a combinedtelephone-telegraphy communication system which is simple inconstruction and economical to manufacture.

Another object of the present invention is to provide a combinedtelephone-telegraphy system which operates with a single wire circuitthereby rendering it easy for installation and maintenance.

A still further object of the present invention is to provide atelephone-telegraphy communication system for fire alarm systems inwhich coded fire alarm signals carry priority over voice communicationsso that the voice communications never interfere with or delay firealarm reception.

A still further object of the present invention is to provide a combinedtelephone-telegraphy system which may be easily added to existing firealarm circuits for a small additional cost.

A still further object of the present invention is to provide a combinedtelephone-telegraphy system which may be used in conjunction with a firealarm circuit for providing a police call system utilizing the existingfacilities of a fire alarm system.

A still further object of the present invention is to provide in a firealarm system a telephonetelegraphy system in which each of the firealarm boxes is provided With telephone capabilities and in which uponremoval of the telephone handset from its cradle an indication of thebox location is displayed at the central station of the fire alarmsystem.

These and other objects and advantages of the invention will becomeapparent from the following description of the preferred embodiment ofthe invention as read in connection with the accompanying drawings inwhich:

FIGURE 1 is a system block diagram illustrating a plurality of seriesconnected fire alarm boxes and a central station;

FIGURE 2 is a combined schematic block diagram illustrating thecircuitry within each of the fire alarm boxes;

FIGURES 3 and 3A are combined schematic block diagrams illustrating thecircuitry at the central station; and,

FIGURE 4 is a schematic circuit diagram illustrating an AND circuitaccording to the present invention.

Referring now to the drawings and more particularly to FIGURE 1, thereis illustrated in block diagram form one preferred embodiment of theinvention as applied to a fire alarm system including a plurality oflocal fire alarm boxes 1, 2, 3 through N, connected together in seriesacross a central station power supply source via central stationequipment 12. The central station power supply source 10 and stationequipment 12 may be lo cated, for example, at the fire departmentheadquarters and the various local boxes 1 through N at desiredlocations within the fire alarm system. Although new equipment may beutilized it is within the contemplation of the present invention thatthe fire alarm boxes 1 through N represent existing equipment within afire alarm system connected together in series by means of a singlemetallic circuit 14 in which direct current flows in accordance with thearrows illustrated in FIGURE 1, having a constant value in the order ofmilliamperes supplied by the power supply source 10. As will bedescribed in greater detail hereinafter, each of the local fire alarmboxes 1 through N includes means for transmitting a distinctive complexfrequency signal, identifying the particular box to the central officeequipment 12, as well as means for transmitting voice frequencies fromthe box to the central office equipment 12. A capacitor 16 is connectedacross the power supply source 10 to prevent the coded and voicefrequency signals from being coupled to the power supply source and itsrelated central station equipment. The central station equipment 12connected in series with the fire alarm boxes 1 through N includessignal separator circuitry 18 which serves to separate the receivedvoice frequency signals from the received coded distinctive frequencysignals. The voice frequency signals are coupled to a voice receiver 20and the coded frequency signals are coupled to box identifying equipment22. The box identifying equipment 22 includes a plurality of decoders 24for decoding the received coded signal and applying the output thereofto suitable logic circuitry 26, which in turn serves to energize anappropriate indicator among the indicators 28 for identifying thecalling box, as well as to provide an output signal to accessoryequipment, such as a recoder 30 for purposes of recording which boxplaced a call. In addition, the signal separator circuitry 18 serves tosupply an output signal to energize suitable equipment 32 for purposesof transferring the voice communications to a remote location viasuitable calling equipment 34. The central station equipment 12 alsoincludes voice transmitting means 36 for purposes of providing two wayvoice communications with the calling box.

LOCAL FIRE ALARM BOXES Having briefily described the general arrangementof the inventive telephone-telegraphy system according to the presentinvention, attention is now directed toward the equipment includedwithin each of the various local fire alarm boxes 1 through N asillustrated in FIGURE 2. Each of the local fire alarm boxes includestelephone equipment for voice communication with the central station 12,as well as coded telegraphy equipment for sending a coded distinctivesignal identifying the box t the central station 12. The telephonecapabilities include a conventional telephone transmitter 38 and aconventional telephone receiver 40. The coded telegraphy equipmentwithin each box includes an encoding unit 42 and a time delay circuit 44which serves to provide power to the encoding unit 42 for a short periodin the order of four to six seconds during which the encoding unit isenergized and serves to transmit coded signals to the central station12. In addition to the foregoing, each of the fire alarm boxes 1 throughN includes circuit means including a power supply circuit 46 forsupplying necessary power to operate the time delay unit 44, theencoding unit 42, as well as transmitter 38. A normally closed shortingswitch S is connected in series with the metallic series loop circuit 14for normally rendering the components within the box inactive.

Having briefly described each of the various components within each firealarm box 1 through N, attention is now directed toward a deaileddescription thereof.

POWER SUPPLY CIRCUIT The power supply circuit 46 is connected across thenormally closed shorting switch S via Winding 48 of a transformer 50.The power supply circuit 46 includes a steering diode 52 connected inseries with the Zener diode 54 across a second steering diode 56, whichin tul'n is connected in parallel with the normally closed switch S viawinding 48 of transformer 50. With direct current flow in the metalliccircuit 14 of the fire alarm system.

in the direction indicated by the arrows i, one side of switch S whenopened may be considered as a positive terminal a and the other side maybe considered as a negative terminal 1). Accordingly, for current flowin the direction indicated by arrows i the anodes of Zener diode 54 andsteering diode 56 are connected directly with the negative terminal band the anode of steering diode 52 and the cathode of steering diode 56are connected to the positive terminal a via winding 48 of transformer50. In this manner, with switch S opened, the Zener diode 54 will berendered reverse biased whereby a positive potential will exist atterminal c, i.e., the junction of the cathodes of stering diode 52 andZener diode 54, with respect to the negative terminal b. With currentflow in the order of 100 milliamperes, as in the case of =1 fire alarmsystem, this potential may be in the order of 5.1 volts.

The time delay circuit 44 includes a unijunction transistor 58 havingits base B1 connected with the negative terminal b via a currentlimiting resistor 60 and its base B2 connected to the positive terminala via current limiting resistor 62 and winding 48 of transformer 50. Theemitter 64 of unijunction transistor 58 is connected to the junction ofa current limiting resistor 66 and a capacitor 68, which are in turnconnected together in Series across the normally closed switch S viawinding 48 of transformer 50. A load resistor 70 is connected inparallel with current limiting resistor 60 via a steering diode 72. Theload resistor 70 serves to provide a positive potential with respect tothe negative terminal b at the gate 74 of a silicon controlled rectifierSCR 76. The silicon controlled rectifier SCR 76 is connected across thenormally closed switch S, as illustrated in FIGURE 2, with its cathode78 connected directly with the negative terminal b and it anode 80connected with the positive terminal a via a current limiting resistor82 and the winding 48 of transformer 50. The junction of SCR 76 andcurrent limiting resistor 82 is connected with the base 84 of an on-offswitching means such as N-P-N transistor 86 via a current limitingresistor 88. Transistor 86 has its emitter 90 connected with thenegative terminal b via a steering diode 92 poled as shown in FIGURE 2.

The encoding unit 42 in each of the fire alarm boxes 1 through Nincludes a pair of resonant reed, solid state oscillators 96, 98 eachserving to develop an output frequency in the range from 250 to 450cycles per second. The output frequencies developed by the twooscillators 96, 98 are different frequencies and, accordingly, theoutput of oscillator 96 may be designated as frequency signal f and theoutput signal developed by oscillator 98 may be designated as frequencysignal f Adjustment of positive feedback for the oscillators 96, 98 iscontrolled by variable resistors 100, 102, respectively, connected tooscillators 96, 98. The output frequency signals developed byoscillators 96, 98 are fed to a dual channel buffer amplifier, includingsingle stage emitter follower amplifiers 104, 106 connected to theoutput circuits of oscillators '96, 98, respectively. The use of emitterfollower circuitry for amplifiers 104, 106 serves the purpose ofpresenting a high impedance input to each of the two oscillators 96, 98.The output circuits of amplifiers 104, 106 are connected to variableresistors 108, 110 respectively, each of which exhibits a greatresistance value compared to the low output impedance of its associatedemitter follower circuitry amplifier. The variable resistor 108, 110performs two functions: (1) they control the amplitude of the outgoingoscillator signal; and (2) serve to isolate oscillators 96, 98 from eachother. Further, the variable resistors 108, 110 serve as gain controlsin the output circuit of each of the amplifiers 104, 106 for adjustingthe magnitude of the output signals at desired levels which are normallydictated by the transmission line and system attenuationcharacteristics. The resistors 108, are connected together at 112whereby the output frequency signals f and f are combined, and fed bymeans of an encoder output lead 114 to one winding 116 of a transformer118. Operating and bias power for the oscillators and amplifiers of theencoder unit 42 is obtained from the output side, i.e., terminals 120,122 of the time delay circuit 44. Output terminal 120 serves as a pointof positive potential as it is connected with the positive terminal avia winding 48 of transformer 50 and output terminal 122 serves as apoint of negative potential with respect to terminal 20 when transistor86 is conductive via the collector 94 to emitter 90 electrodes of thetransistor and steering diode 92 to the negative terminal 12. As can beseen with refer ence to FIGURE 2, negative bias potential is obtainedfrom the negative output terminal 122 for providing appropriate biaspotential for oscillators 96, 98, as well as the amplifiers 104, 106.Further, operating positive potential is obtained from the positiveoutput terminal 120 for providing suitable operating power foroscillators 96, 98 as well as the amplifiers 104, 106.

The output circuit of the voice transmitter 38 is connected to the highpass filter 121 which presents a high impedance to all frequencies below600 cycles per second and passes all frequencies above 600 cycles persecond. The output circuit of high pass filter 121 is in turn connectedacross a loading resistor 123, as well as across the winding 116 oftransformer 118. A blocking capacitor 124 is incorporated in the filter121 and serves to block low frequency signals thereby preventing theoutput signals of oscillators 96, 98 from being unduly attenuated by thenormally low impedance presented at the output circuit of filter 121.The other side of transformer 118, i.e., winding 126, couples the outputcircuit of filter 121, as well as the output circuit 114 of the encodingunit 42 with a common power amplifier 128. The amplifier 128 may be ofconventional design but is preferably a solid state, push-pull poweramplifier. Bias potential for the base emitter circuitry of amplifier128 is obtained from output terminal 0 of the power supply circuit 46via a mid-tap on the winding 126 of transformer 118. Negative biaspotential for the amplifier 128 is obtained from the negative terminalb. The operating potential for amplifier 128 is obtained from outputterminal 120 of the time delay circuit 44. The amplifier 128 serves adual function in that it provides power amplification for both the voicetransmitter 38, as well as for the encoding circuit 42. The outputcircuit of amplifier 128 is coupled to the fire alarm metallic circuit14 via windings 49, 48 of the transformer 50.

A filter 130 is connected across winding 48 of transformer 50 and hasits output circuit connected across the input circuit of the telephonereceiver 40. Filter 130 is a high pass filter presenting a highimpedance to frequencies below 600 cycles per second and passing to thereceiver 40 only those signals of a frequency in excess of 600 cyclesper second.

CENTRAL STATION EQUIPMENT The various equipment located at the centralstation 12 together with the central ofiice power supply 10 described ingeneral terms with respect to FIGURE 1, will hereinafter be described ingreater detail with reference to the combined schematic block diagramillustrated in FIG- URE 3.

The voice transmitter 36 at the central station 12 has its outputcircuit coupled to a high pass filter 132 which presents a highimpedance to signal frequencies below 600 cycles per second and servesto pass frequencies above 600 cycles per second. The output circuit ofthe high pass filter 132 is coupled across the input circuit of a voiceamplifier 134 via windings 136, 138 of transformer 140. Amplifier 134may be of conventional design but preferably is of solid state circuitrytaking the form of a pushpull class A amplifier. The output circuit ofthe amplifier 134 is in turn coupled to the separator circuitry 18 viawindings 142, 144 of a transformer 146.

A power supply circuit 145 is included at the central station 12 andserves to provide bias and operating potentials to the voice transmittercircuitry, as well as circuitry to be described hereinafter. The powersupply circuit 145 includes a direct current voltage supply sourceconnected between output terminals 147, 149 of the power supply circuit.The supply source may take the form of battery which preferably is of 24volts positive potential at terminal 147. A resistor 153 and a Zenerdiode 155, poled as shown in FIGURE 3A, are connected together in seriesacross battery 151 so that the Zener diode is rendered reverse biasedand serves to provide a regulated voltage potential at output terminal157 taken from the junction of the resistor 153 and the cathode of theZener diode 155. A steering diode 159, poled as illustrated in FIGURE3A, is connected in series with another resistor 161 across the Zenerdiode 155 so that a third potential with respect to terminal 157 isobtained at output terminal 163 taken from the junction of the resistor161. and the cathode of diode 159. As illustrated in FIGURE 3A, bias andoperating potentials are obtained from terminals 157 and 163 andsuitably applied to the voice transmitter 36, as well as filter 132 andthe voice amplifier 134.

The signal separation circuit 18 serves the function of separatingincoming box identifying signals from incoming voice frequency signalsand channeling each to its proper destination, as well as channeling theoutgoing voice frequency signals to the fire alarm metallic circuit 14while inhibiting such signals from reaching either the voice receiver orthe input of the decoder circuitry 24. Signal separation circuit 18includes a pair of transformers 148 and 156, each having three windingsof one to one turn ratio, i.e., transformer 148 has windings 152, 154and 156 and transformer has windings 158, and 162. Windings 154 and 160are connected together in series across winding 144 on transformer 146.Windings 152, 158 are connected together in series opposition so that,as is well known in the art, if equal voltages are developed across thetwo windings as in the case if the load seen by each transformer 148 and151) are equal in magnitude and phase, the voltages will cancel. Toaccomplish this effect a loading resistor 164 is connected acrosswinding 162 on transformer 150 and a variable resistor 166 is connectedacross winding 156 on transformer 148 for purposes of balancing thecombination load of resistor 164 across winding 162 on transformer 150,as well as any shunting line resistance present in the metallic circuit14. Further, a variable balancing capacitor 168 is connected acrosswinding 156 on transformer 148 for purposes of balancing the effectiveline capacity shunting the winding 162 on transformer 150. The signalseparation circuit 18 also serves to bypass other fire alarm equipmentassociated with the 100 milliampere direct current voltage supply source10 by virtue of the bypass capacitor 16 connected across the output ofthe supply 10. With the foregoing circuitry, output voice frequenciesfrom the voice transmitter 36 will result in equal and opposite voltagesinduced across windings 152, 158, which will cancel and, hence, notreach either the decoder inputs to the decoder circuitry 24 or the inputof voice receiver 20. However, the voice frequency output signal, asamplified by amplifier 134, will be developed across output winding 162of transformer 15d and thereby impressed on the fire alarm metalliccircuit 14.

A pair of filters 170, 172 are connected together across windings 152,158 of transformer 150. A resistor 174 is connected in shunt acrossfilter 172 from the junction of filters 170, 172 to the winding 158.Filter 172 is a high pass filter presenting a high impedance to signalsof a frequency below 600 cycles per second and passing only thosefrequencies above 600 cycles per second. The output of filter 172 iscoupled across the input of voice receiver 28*. Filter is a low passfilter which presents a high impedance to signals of a frequency inexcess of 450 cycles per second, and passes only those signals of afrequency below 450 cycles per second. The output circuit of filter 170is coupled to the input circuit of a filter 176. Filter 176 is a highpass filter presenting a high impedance to frequencies below 250 cyclesper second and passing those si nal frequencies above 250 cycles persecond. Therefore, filters 170 and 176 serve as a band pass filter forpassing signals in the frequency range of 250 cycles per second to 450cycles per second. This is the frequency range of the output signalsappearing in the output circuit 114 of the encoding unit 42 locatedwithin each of the local fire alarm boxes 1 through N.

The output circuit of filter 176 is applied across the input terminalsof each decoding circuit 1 through N in the decoding unit 24 located atthe central station 12. Each of the decoders 1 through N is a frequencyselector, preferably of solid state circuitry, which serves to produce aDC voltage output signal only when fed by an AC signal of a selectedfrequency. Thus, for example, the de;oders may take the form of resonantreed relays preceded by several stages of clipper amplifiers and foreach oscillator 96, 98 in the local traffic boxes 1 through N, therewill be one decoder circuit which will respond to that and only thatfrequency with a maximum band width of plus or minus 2% and a minimumband width of plus or minus 25%. Thus, since two oscillators 96, 98 areutilized in each of the local fire alarm boxes the composite outputsignal developed at the output circuit 114 of each of the encoding units42 will activate two decoder circuits 1 through N in the decoder unit 24at the central station 12. The outputs of the decoder circuits 1 throughN, i.e., outputs 1 through N, represent DC voltages and are fed toseveral AND circuits which comprise the logic circuit 26, with ANDcircuits 178, 180, 182 and 184 only being illustrated in FIGURE 3A. Thetotal number of decoder circuits N utilized may be found from thefollowing expression:

AND circuits where the number of AND circuits is equal to the number oflocal fire alarm boxes in the system.

Two decoder outputs are fed to the input circuit of each AND circuit.Any particular decoder output may be fed commonly to several of the ANDcircuits, but any particular combination of two decoder outputs is fedto only one AND circuit. Thus, for example, as illustrated in FIGURE 3A,AND circuit 178 has inputs received from decoders 1 and 2, AND circuit180 has inputs received from decoders 1 and 3, AND circuit 182 hasinputs received from decoders 2 and 3, and AND circuit 184 has inputsreceived from decoders 1 and N.

A reset circuit 191 is provided for simultaneously resetting all of theAND circuits associated with the logic circuitry 26 and as illustratedin FIGURE 3A includes a normally closed switch S2 connected to theoutput terminal 147 of the power supply source 145 and to inputterminals 186, 188, and 192 of the AND circuits 178, 180, 182 and 184,respectively. The reset circuit 191 together with its interconnectionwith each of the AND circuits may be more clearly understood withreference to FIGURE 4 which is a schematic diagram of U AND circuit 184and partially illustrating decoding circuits 1 and N. It will be notedwith reference to FIGURE 3A that resistors 194, 196, 198 and 200 areconnected between terminal 149 of power source 145 and the outputcircuit of decoding units 1 through N. These resistors as illustrated inFiG-URE 4 also serve as a portion of the AND circuits 178, 186, 182 and184. In FIGURE 4 the decoding units 1 and N are illustrated forschematic purposes as comprising switches 202 and 2114. Switch 202 isillustrated as comprising a movable contact 206 connected to terminal149 via resistor 194 and a stationary contact 208 connected with outputterminal 147 of the power supply circuit 145. Similarly, switch 204 isillustrated as comprising a movable contact 1210 connected to terminal149 via resistor 200 and a stationary contact 212 connected to theoutput terminal 147 of the power supply circuit 145. The AND circuit 184includes a silicon controlled rectifier SCR 214 having its gate 216connected through the cathode to anode circuit of a steering diode 218to the junction of resistor 194 and movable armature 206 via the anodeto cathode circuit of a steering diode 220, and to the junction ofmovable armature 210 and resistor 299 via the anode to cathode circuitof a steering diode 222. The reset circuit 19'1 is connected from theoutput terminal 147 of the power supply circuit 145 via reset switch S2to the anode 224 of SCR 214 via a current limiting resistor 226 and tothe cathode 228 of the SCR 214 via a current limiting resistor 230. Theanode 224 of SCR 214 is also connected with the junction of the anodesof steering diodes 218, 220 and 222 via resistor 226 and a resistor23-2. A resistor 23-4 is connected between terminal 149 and the gate 216of silicon controlled rectifier SCR 214 which resistor is of greaterresistance than either of the resistors 194, 196, 198 and 200, which areeach of equal resistance and further which resistance is of smallerresistance than resistor 232, the purpose of which will be described ingreater detail hereinafter with reference to the description ofoperation. The cathode 228 of SCR .214 is also connected to terminal 149via a loading resistor 236 connected across a series circuit includinglamp LN and a current limiting resistor 238. A capacitor 24(i-isconnected between the gate 216 and the cathode 228 of SCR 214 forpurposes of protecting against radio frequency triggering effects.Further, another capacitor 242 is connected between terminal 149 and theanode 224 of SCR 214 for purposes of protecting the circuit against rateeffects. Rate effect is described on pages 26 and 27 of the GeneralElectric SCR Manual, second edition. It essentially relates to reducingrise of voltage as applied between the anode and cathode of a SCR.

The indicator portion 28 at the central station 12 includes anindividual visual box identifier for each of the local fire alarm boxes1 through N for displaying an indication as to which particular box senttwo unique frequencies which activated a particular pair of decoderswhich in turn energized a particular indicator. These indicators takethe form of lamps L1 through LN, as illustrated in FIGURE 3A, with lampL1 being present in the output circuit of AND circuit 178, L2 beingpresent in the output circuit of AND circuit 180, lamp L3 being presentin the output circuit of AND circuit 1-82, and the lamp LN being presentin the output circuit of AND circuit 184. With reference to FIGURE 4, itwill be noted that each of the lamps L1 through LN is connected to itsassociated AND circuit in the manner in which lamp LN is connected, asillustrated in FIGURE 4. Each of the lamps L 1 through LN is connectedwith a common indicator 244 which includes both an audio alarm deviceand a suitable lamp whereby upon any of the lamps I11 through LN beingenergized, so also will the common indicator 244.

'If it is desired to provide a permanent recording as to which of thelocal fire alarm boxes 1 through N placed a call to the central station12, a suitable console recorder 246 may be connected to the outputcircuits of lamps L1 through LN. This additional circuitry includes aplurality of triggered switches 24 8, 250, 252 and 254, respectively,connected to the output circuits of lamps L1 through LN. The outputcircuits of the triggered switches v248 through 254 are in turnconnected to a coding matrix 256, as well as to a gated stepping switchand driver 258. The output circuit of the coding matrix 256, as Well asthe output circuit of the gated stepping switch and driver 25 8, areconnected to a suitable stepping switch device 260. Eurther, an ANDgating circuit 262 is connected to outputs taken from the steppingswitch device 260, as well as the gated stepping switch and driver 258,whereupon if inputs are received from both then an output signal will betransmitted to the input of the console recorder 246.

OPERATION OF EACH INDIVIDUAL FIRE ALARM BOX In operation of the combinedtelephone-telegraphy system according to the present invention, a codeddistinctive signal representative of a particular fire alarm box istransmitted and relayed over the single metallic circuit 14 upon openingthe normally closed switch S (see FIG- URE 2). Preferably, and withinthe contemplation of the present invention, the normally closed switch Srepresents a telephone handset cradle and that the handset incorporatesboth the transmitter 38, as well as the receiver 40. The person placingthe call merely removes the handset from the cradle and the resilientlybiased switch S opens, as represented by the dotted lines in FIGURE 2.With switch S in its open position the short circuit shunt betweenterminals :1 and b is removed whereby current may flow from the positiveterminal a through the winding 48 of transformer 50 to the power supplycircuit 46 so that the Zener diode 54 is rendered reverse biased byvirtue of the direction of current flow and the configuration of thesteering diodes 5-2, 56. Accordingly, suitable bias supply is obtainedat terminal 0 of the power supply for purposes of supp-lying baseemitter bias for the transistor power amplifier 128 via mid-tap 130 onwinding 126 of transformer 118. Current will also flow from positiveterminal a through the winding 48 of transformer to the biasingresistors 8-2, 8-8, the base 84 to emitter of transistor 86, steeringdiode 22 to the negative terminal b thereby forward biasing transistor86. In this manner, transistor 86 is heavily biased conductive andessentially a short circuit is obtained between its output terminal 122and the negative terminal b. Accordingly, a potential difference willexist between the output terminals 128 and 122 of the time delay circuit4 4 for purposes of providing operating and biasing potential to theoscillators 96, 9'8 and their associated amplifiers 1G4, 106,respectively. Oscillator 96 will develop an output frequency signal h,which is suitably amplified by the amplifier 184' and applied to theoutput circuit 114. Similarly, oscillator 98 will develop an outputfrequency signal f which is suitably amplified by amplifier 106, and,likewise, applied to the output circuit 114. The composite output signalpresent in the output circuit 114 is a complex wave form being a mixtureof amplified output frequency signals f f and is applied to the input ofpower amplifier 128 via transformer 118. The coded frequency signaldeveloped by the encoder unit 42 is thus amplified by the poweramplifier 128 and coupled to the fire alarm metallic circuit 14 via thecoupling transformer 50 and thence to the central station 12. Thedirection or arrow i in FIGURE 2 indicates the direction of themilliampere direct current that normally flows through the metalliccircuit (see FIGURE 1). The coded frequency signal impressed acrosstransformer winding 48, in FIG- URE 2, is an alternating current voltageand therefore will alternate in direction.

During the period in which the encoder unit 42 is transmitting a codedfrequency signal, timing capacitor 68 in the time delay circuit 44 ischarged through its associated timing resistor 66. When the capacitor 68is sufficiently charged that the potential existing between the negativeterminal b and emitter 64 of unijunction transistor 58 reaches thecharacteristic peak point voltage of the unijunction transistor, thenthe emitter 64 will become forward biased and the dynamic resistancebetween the emitter "64 and base B1 will drop to an exceedingly smallvalue. The capacitor 58 will then discharge through the emitter 64, baseB1 of unijunction transistor 58 through the load resistor 60. Thepotential existing across the load resistor 60 will be coupled throughthe steering diode 72 and appear across a second loading resistor 70.The potential eixs-ting across loading resistor 70 serves as a positivegating potential of sufficient magnitude to forward bias the siliconcontrolled rectifier SCR 76 when applied to the gate 74. Accordingly,the silicon controlled rectifier SCR 76 becomes conductive andessentially short circuits the base to emitter electrodes of transistor86. In this manner, transistor 86 becomes nonconductive and thepotential existing between the output terminals 120, 122 of the timedelay circuit 4-4 no longer exists. Thus, operating and bias potentialfor the oscillators 9'6, 98 and their asociated amplifiers 164, 1106,respectively, is removed whereby the encoding unit 42 ceases to transmita coded frequency signal. Accordingly, the unijunction transistor 58 andthe SCR 76 may be defined as control means for turning transistor 86 offwhen the magnitude of energy stored by the energy storage means, i.e.,capacitor 68, reaches a predetermined value. As can be appreciated, thetime involved during which the encoding unit 42 transmits a coded Signalto the completion thereof, is controlled by the RC timing circuitcomprising resistor 66 and capacitor 68 of the time delay circuit 44.Preferably, resistor 66 and capacitor 68 are choosen so that theencoding unit 42 transmits a coded frequency signal for a period of fourto six seconds. The user of the telephone handset at the local firealarm box placing the call will hear a tone signal for the duration ofthe transmitting period of encoding unit 42, at the completion of whichthe user may commence voice communications with the voice communicationsystem at the central station 12.

Voice communications with the central station 12 are provided by thevoice frequency transmitter 38, which transmits voice frequency signalsvia the high pass filter 121 and coupling transformer 118 through thepower amplifier 128 to the metallic fire alarm circuit 14 via a secondcoupling transformer 59. The user of the telephone communication systemat the local fire alarm box receives voice communications by virtue ofthe voice receiver 40* connected across winding 48 of transformer 50 viathe high pass filter 130.

OPERATION OF THE CENTRAL STATION EQUIPMENT In the operation of theequipment at the central station 12 coded box identifying frequencysignals are re ceived over the metallic interconnecting circuit 14 bythe signal separator circuitry 18 as the signals are applied acrosswinding 162 of transformer 150. Since all of the windings, i.e., 158,160, 162 on the transformer 150 are of one to one turn ratio, equalvoltages are developed across windings 160, 158. Since winding 154 ontransformer 148 is in series with winding 160 on transformer 150, thevoltage induced across winding 160 will cause sutlicient current flowthrough winding 154 to develop a voltage drop across winding 154. Sincethe windings 152, 154, 156 on transformer 148 are of one to one turnratio, an equal voltage will also be developed across winding 152, whichdue to the winding polarities will serve to aid the voltage developedacross 158 on transformer 150.

To simplify the explanation, place dots on the transformers to indicatepolarity; specifically place dots at the top of windings 158, 160 and162 of transformer 150 and also at the top of windings 148 and 156 oftransformer 154. Place a dot at the bottom of winding 152 of transformer154. The dot signifies that if a voltage is impressed across a windingof the transformer of such polarity that the end of the winding with thedot is positive with respect to the other end; all other windings on thesame transformer core will be polarized likewise. Therefore, if winding162 of transformer 150 is polarized with a signal from the line,windings 160 and 153 will be polarized the same. Since winding 148 oftransformer 154 is serially connected and acts as a load along with thereflected impedance of transformer 146 to the inducted source voltage ofwinding 160 of transformer 150, the polarity of the voltage in Winding148 of transformer 150 will be opposite that of winding 158 oftransformer 154. This means the dot end of winding 148 as well as 152 oftransformer 154 will be negative if the dot winding of of transformer151) is positive. Therefore, winding 152 of transformer 154 and winding158 of transformer 160 are oppositely polarized; but since they areconnected in series opposition, the voltages will aid. Since the inputimpedance of the high pass filter 172 will be high with respect to thelow frequency (250 to 450 cycles per second) of the coded voicefrequency signal, the resistor 174 will act as the effective loadingresistance and the effective load presented to winding 15% oftransformer 150 will be the series load of the effective load on winding152 of transformer 148, and the input impedance of the 450 cycles persecond low pass frequency filter and the loading resistor 174. Anyportion of the coded frequency signal developed across the 600 cycle persecond high pass filter 172 will be dissipated by the filter as it doesnot pass these low frequency signals, whereas that portion of the codedfrequency developed across the low pass 450 cycles per second filterwill readily be passed therethrough. The output signals appearing on theoutput circuit of filter 171) are in turn directly coupled to the 250cycle per second high pass filter 176, which serves to reject allfrequencies below 250 cycles and pass all frequencies thereabove. Inthis manner, any noise pick-up signal, such as 60, 120 or cycles persecond, will not be passed. Thus, it is seen that filters 170 and 176serve as a band pass filtering device for purposes of passing codedsignal box identifying frequency signals which are in the range of 250to 450 cycles per second. The output signals developed across the outputof filter 176 are thence fed to the input circuits of decoder units 1through N of the decoder circuitry 24. As stated hereinbefore, thedecoders 1 through N are each resonant reed relays preceded by severalstages of clipper amplifiers thereby rendering the decoders sensitive toa very narrow frequency range and more specifically they are matched tothe resonant reed oscillators found in the encoder units 42 at thevarious local fire alarm boxes. The number of decoders will depend onthe number of fire alarm bxes utilized in the system and for eachfrequency f f etc., used by the fire alarm boxes there will be onedecoder which will respond to that particular frequency and provide a DCoutput voltage on its output circuit which is thereafter fed to the ANDcircuits within the logic circuitry 26.

Assuming that the particular fire alarm box which is sending a codedcomplex frequency signal to the central station, includes two encodingoscillators one of which develops a frequency signal 1, and the otherdevelops a frequency signal f corresponding respectively to thefrequency response characteristics of decoding units 1 and N, then onlyrecoding units 1 and N will have an output signal present at theiroutput terminals. With reference to FIGURE 3A, it will be noted that theonly AND circuit that is responsive to input signals developed bydecoders 1 and N for in turn providing an output signal is AND circuit184. The operation of AND circuit 184 will be better understood withreference to the schematic diagram shown in FIGURE 4.

Prior to the activation of the decoder circuits 1 and N, i.e., whenswitches 202, 204 are open as illustrated in FIGURE 4, current will flowfrom the positive terminal 147 of the voltage supply source 145 throughthe normally closed reset switch S2, resistor 232 and thence throughparallel paths comprising diode 220 and resistor 194 in parallel withdiode 222 and resistor 200 to terminal 149. Current will also flow fromthe positive terminal 147 to terminal 149 through resistor 232, steeringdiode 213 and resistor 234. As stated hereinbefore, the resistancevalues of resistor 194, 20t), 232 and 234 are such that resistor 234 isof greater resistance than either of the resistors 194 or 2%, but lessthan that of resistor 232 and hence the potential existing at gate 216of silicon controlled rectifier SCR 124 is somewhat low. Further, theresistors 230, 236 and 238 and lamp LN form a voltage divider wherebythe potential existing at the cathode 228 of SCR 214 is at a higherpotential than that existing at the gate 216. Consequently, the siliconcontrolled rectifier SCR 214 is reverse biased and lamp LN will not beenergized, i.e., the value of the resistance 230 is sufiiciently highthat current flow from the positive terminal 147 to terminal 149 throughresistor 230, resistor 238 and lamp LN will be insufficient to causelamp LN to glow. The capacitor 242 serves to protect the circuit againstrate effects and the capacitor 24%) serves to protect the circuitagainst radio frequency triggering. However, when the two decoders 1 andN are energized, i.e., with switches 204 and 206 closed, current willflow therethrough from the positive terminal 147 and sufiicientpotentials will be developed across resistors 194, 209 so that thediodes 220, 222 will be rendered reverse biased. Accordingly, thepotential existing at gate 216 of silicon controlled rectifier SCR 214will assume a higher value determined by the voltage divider circuit nowconsisting of resistor 232, diode 218 and resistor 234. This potentialwill be sufficient to forward bias the SCR 214, which will then conductand shunt the resistor 230 (i.e., resistor 226 is of smaller resistancethan resistor 230). The current flow from the positive terminal 147through the reset circuit 191, resistor 226, the anode to cathodecircuit of SCR 214, resistor 238 and lamp LN to terminal 149 will be ofsufiicient magnitude to cause lamp LN to glow brightly. The fact thatlamp LN glows brightly indicates to the observer at the central station12 that the particular fire alarm box which produces a complex outputfrequency signal made up of frequency signals 1, and f is placing acall. The observer at the central station at this time, or after asufiicient period to permit the console recorder 246 to record theparticular fire alarm box which has been identified, may reset all ofthe AND circuits and the logic circuitry 26 by merely opening thenormally closed reset switch S2, thereby removing current from the SCR214 and unlatching it. It will be noted with reference to FIGURE 3A thatthe reset switch S2 in the reset circuit 11 is common to all of the ANDcircuits and, hence, all of the AND circuits will be reset upon openingof switch S2.

In addition to providing visual indication that a particular fire alarmbox has placed a call, each of the visual box identifier lamps L1through LN, produces an output signal which is fed to a common audio andvisual monitor circuit 244. Thus, the actuation of any of the lampcircuits L1 through LN effectively turns on the monitor 244 to sound abuzzer and/or a common lamp. In addition to the foregoing outputs, eachof the visual box indicator lamps 1 through LN also provides an outputsignal which is fed to triggered circuits 248 through 254, respectively.The output of each triggered switch circuit 248 to 254 is in turn fed toboth a coding matrix circuit 256 and a gated stepping switch andrecorder driver 258. A suitable stepping switch 260 is connected to theoutput circuits of the coding matrix device 256 and the gated steppingswitch and driver circuit 258 for purposes of providing a gating signalto a suitable gate 262. The gate 262 in turn provides an output signalto the console recorder 246 only in the absence of an input signal fromthe stepping switch 260. In the event of no input signal from thestepping switch 260 the recorder 246 would print eleven marks. By theuse of the coding matrix 256 any one or any number of the last ten markscan be eliminated, thus forming a digit which is an indication that aparticular fire alarm box has placed a call.

Voice communications with the calling fire alarm box are provided at thecentral station by means of a telephone transmitter 36 and the voicereceiver 20. The output voice frequency signals developed by thetransmitter 36 are coupled to a voice amplifier 134 via a high passfilter 132 and transformer 140. The amplified voice frequency signals,which due to the filter action of high pass filter 132 are above 600cycles per second, are coupled across transformer 146 and supplied tothe metallic fire alarm interconnecting circuit 14 by way of winding 162on transformer 150. Incoming voice frequency signals from the callingfire alarm box are developed across winding 162 of transformer 150 andsince all of the windings on transformer 150 have a one to one turnratio, equal voltages are developed across windings 158 and 160 equal tothat across winding 162. The load presented to the winding 160 ontransformer 150 is a series load consisting of the effective load oftransformers 148 and 146. However, the effective load of transformer 146is essentially infinite compared to that of transformer 148 since theformer represents the impedance reflection of the pushpull transistoramplifier 134. Thus, in essence, current flow through windings 154 and160, transformers 148, 156, respectively, is limited by the magnetizinginductance of transformer 146 and most of the voltage developed acrosswinding 160 on transformer 150 will be impressed across transformer 146and very little across transformer 148. The load presented to winding158 on transformer 150 is the effective load presented to Winding 152 ontransformer 148 in series with the input impedances of filters and 172shunted by the loading resistor 174. Thus, the loading at voicefrequencies on winding 152 of transformer 148 is efiectively the valueof resistance 166 and the input impedance of filter 170, which are lowcompared with the input impedance of filter 172 shunted by the loadingresistor 174. Thus, a good portion of the incoming voice frequencysignal is developed across the high pass filter 172. Since the incomingvoice frequency signal contains no frequencies below 600 cycles persecond, it is seen that the portion of the voice frequency signal whichis developed across the low pass 450 cycles per second filter 170 isdissipated therein and will not reach the decoders 1 through N of thedecoder circuitry 24. Accordingly, the voice frequency signals which aredeveloped across the 600 cycles per second high pass filter 172 arereadily passed through the filter to the voice receiver 20.

The fire alarm boxes, with the disclosed circuitry employed, will sendan alarm signal as long as the DC. line current does not drop belowapproximately 40 ma. To determine the number of boxes which may operatesimultaneously it is necessary to know the total line resistance and thesource voltage as well as the total voltage drop across each box. In thepreferred application of this system, the supply voltage is 110 v., themaximum line resistance is 500 ohms, and the voltage drop across eachbox is 6 volts maximum (constant regardless of current). The formula forcalculating the maximum number of boxes which may send in an alarmsimultaneously is:

V =source voltage N =number of boxes R =line resistance in ohms With 110v.=V and R =5OO ohms =15 boxes We claim:

1. A combined telephone-telegraphy system comprising a plurality oflocal calling boxes and central station communication equipmentconnected together in a series circuit and adapted to be connectedacross a power source for maintaining current flow in said seriescircuit, each said local calling box including voice communication meansfor transmitting and receiving voice frequency signals to and from saidcentral station equipment as well as electrically powered electronictelegraphy means for electrically transmitting coded box identifyingtone frequency signals to said central station equipment, each saidlocal calling box including circuit means for providing operatingelectrical power for said local calling box from said current flow insaid series circuit, said central station equipment including voicecommunication means for transmitting and receiving voice frequencysignals to and from said local calling boxes as well as calling boxidentifying means responsive to said coded box identifying tonefrequency signals for identifying the particular calling boxtransmitting said coded box identifying signals, said circuit meansincludes a power supply circuit for providing bias potentials derivedfrom said current fiow in said series circuit for said voicecommunication means and said telegraphy means at said box.

2. A combined telephone-telegraphy system as set forth in claim 1including normally closed switching means for each box shunting saidcircuit means at said box thereby rendering said circuit meansnonresponsive to current flow in said series circuit until saidswitching means is opened.

3. A combined telephone-telegraphy system as set forth in claim 1wherein said telegraphy means at each local calling box includesoscillator means for developing a coded frequency signal representativeof its associated calling box.

4. A combined telephone-telegraphy system as set forth in claim 3wherein said oscillator means includes a pair of oscillators eachserving to develop a signal of a different frequency from that of theother oscillator.

5. A combined telephone-telegraphy system as set forth in claim 1wherein said central station equipment includes signal separationcircuit means for separating received voice frequency signals fromreceived coded box identifying frequency signals and respectivelycoupling said signals to said central station voice communication meansand said calling box identifying means.

6. A combined telephone-telegraphy system comprising a plurality oflocal calling boxes and central station communication equipmentconnected together in a series circuit and adapted to be connectedacross a power source for maintaining current flow in said seriescircuit, each said local calling box including voice communication meansfor transmitting and receiving voice frequency signals to and from saidcentral station equipment as well as telegraphy means for transmittingcoded box identifying frequency signals to said central stationequipment, each said local calling box including circuit means forproviding operating power for said local calling box from said currentflow in said series circuit, said central station equipment includingvoice communication means for transmitting and receiving voice frequencysignals to and from said local calling boxes as well as calling boxidentifying means responsive to said coded box identifying frequencysignals for identifying the particular calling box transmitting saidcoded box identifying signals, normally closed switching means for eachbox shunting said circuit means at said box thereby rendering saidcircuit means nonresponsive to current flow in said series circuit untilsaid switching means is opened, said telegraphy means at each localcalling box includes oscillator means for developing a coded frequencysignal representative of its associated calling box, and time delaycircuit means for de-activating said oscillator means after apredetermined time subsequent to the opening of said normally closedswitching means whereby said coded frequency signals are transmitted foronly a predetermined period.

7. A combined telephone-telegraphy system comprising a plurality oflocal calling boxes and central station communication equipmentconnected together in a series circuit and adapted to be connectedacross a power source for maintaining current flow in said seriescircuit, each said local calling box including voice communication meansfor transmitting and receiving voice frequency signals to and from saidcentral station equipment as well as telegraphy means for transmittingcoded box identifying frequency signals to said central stationequipment, each said local calling box including circuit means forproviding operating power for said local calling box from said currentflow in said series circuit, said central station equipment includingvoice communication means for transmitting and receiving voice frequencysignals to and from said local calling boxes as well as calling boxidentifying means responsive to said coded box identifying frequencysignals for identifying the particular calling box transmitting saidcoded box identifying signals, and common power amplifying means foreach said calling box for amplifying the output of said voicecommunication transmitting means and the output of said telegraphymeans, and coupling circuit means coupling the output of said commonpower amplifying means with said series circuit.

8. A combined telephone-telegraphy system as set forth in claim 7including filtering means interposed between the output of said voicecommunication transmitting means and the input of said common poweramplifying means for passing voice frequency signals to said commonpower amplifying means which exceed a predetermined frequency.

9. A combined telephone-telegraphy system as set forth in claim 8wherein said filtering means includes a blocking capacitor which servesto prevent coded box identifying frequency signals transmitted by saidtelegraphy means which are below said predetermined frequency from beingunduly attenuated.

10. A combined telephone-telegraphy system comprising a plurality oflocal calling boxes and central station communication equipmentconnected together in a series circuit and adapted to be connectedacross a power source for maintaining current flow in said seriescircuit, each said local calling box including voice communication meansfor transmitting and receiving voice frequency signals to and from saidcentral station equipment as Well as telegraphy means for transmittingcoded box identifying frequency signals to said central stationequipment, each said local calling box including circuit means forproviding operating power for said local calling box from said currentflow in said series circuit, said central station equipment includingvoice communication means for transmitting and receiving voice frequencysignals to and from said local calling boxes as well as calling boxidentifying means responsive to said coded box identifying frequencysignals for identifying the particular calling box transmitting saidcoded box identifying signals, said central station equipment includessignal separation circuit means for separating received voice frequencysignals from received coded box identifying frequency signals andrespectively coupling said signals to said central station voicecommunication means and said calling box identifying means, and saidsignal separation circuit means includes high pass voice frequencyfilter means for passing only received signals of a frequency above apredetermined frequency and band pass filter means for passing onlyreceived signals of frequencies within a frequency band below saidpredetermined frequency, the output of said high pass filter means beingcoupled with the input of said voice frequency receiving means at saidcentral station and the output of said band pass filter means beingcoupled with said box identifying means.

11. A combined telephone-telegraphy system as set forth in claim 10wherein said band pass filter means includes a first low pass filter forpassing received signals of frequencies below that of said predeterminedfrequency and a second high pass filter coupled to the output of saidfirst low pass filter for passing signals of a frequency above a secondpredetermined frequency, the output of said second high pass filter beincoupled to said box identifying means.

12. A combined telephone telegraphy system as set forth in claim 10wherein said box identifying means includes a plurality of decoder meanseach responsive to a particular frequency within a frequency range ofsignals passed by said band pass filter means for providing an outputdecoder signal.

13. A combined telephone-telegraphy system as set forth in claim 12wherein said telegraphy means at each said local calling box includes apair of oscillators each serving to develop a signal of a frequencydifferent than that from the other oscillator as well as from theoscillators in said other local calling boxes, decoding means at saidcentral station for each said oscillator among said plurality ofoscillators in said local calling boxes responsive to the frequency ofthe signal developed by one of said oscillators for providing an outputdecoder signal.

14. A combined telephone-telegraphy system as set forth in claim 13including logic circuitry means responsive to various combinations ofsaid output decoder sig nals for determining which local calling box istransmitting coded box identifying frequency signals.

References Cited UNITED STATES PATENTS 1,613,802 1/1927 Mollard 340288 X2,101,209 12/ 1937 Bacon 340-287 2,820,098 1/ 1958 Skogsberg 179-52,887,535 5/1959 Craddock 179-5 ROBERT L. GRIFFIN, Primary Examiner.

20 W. S. FROMMER, Assistant Examiner.

